A lot of the players don't really have a CL mentality, some of them totally under performed (even Lukaku mention it, they just weren't there with the right mentality). It might be an unpopular opinion, but I think that Mourinho idea of bringing Fellaini in place of McTominay is that McTominay is more defensive midfielder, while Fellaini brings the height advantage inside Sevilla own box, bringing more offensive opportunities to United.
A quick view of the game stats, in the first 20 minutes of the game (like Mourinho was saying), things were going decent. Fellaini was the player who won more aerial duels from all United players, with most of those duels around the first 20 minutes:
https://i.imgur.com/SDBsV3S.png
We can also see that Fellaini wasn't really set to defend but to bring more attacking power to United, he had the most Offensive Aerials out of the whole United team:
https://i.imgur.com/cCjY9u2.png
But unfortunately, after those 20 something minutes the team started disappearing. Yes, the tactics didn't pane out like Mourinho wanted, and yes, the exhibition in a whole was poor. Mourinho tactics didn't pane out like he wanted them to, but I can understand the reason behind Fellaini. It's not like Mourinho set the team to wait for Sevilla mistakes or to defend a draw like some even suggest it by mentioning Fellaini. And yes, I'm not defending Fellaini exhibition (it was poor), just pointing it out how Fellaini was set up by Mourinho.
Loads of people here also talking bad about Sanchez in general (not just this game). Yes, Sanchez didn't perform like he's capable off, but are you guys forgetting that he didn't do pre-season? He signed with United and days after it, he was already playing for the team! Some players are capable of adapting faster than others, but it's unfair what some fans talk about Sanchez. In my opinion Sanchez will get better and perform better the more he adapts to United team.
At the end of the game, yes Mourinho tactics didn't pane out and some players totally froze (maybe CL nerves or lack of experience in CL?) but don't forget before this game United actually defeated Liverpool and Chelsea. Don't be so short-minded and forgetting those games because of a really bad game today, people overreact and want to sack the manager and some players like it was the end of the world. United last glorious days are in the past, things have to change and they won't change in a blink of an eye.
About the whistles of United fans against their own team. Another probably unpopular opinion here, but I think it's disgusting doing it when the game is still open at 0-0. I hated when Real Madrid fans do it to their own team, now United seems to be going to the same road... I mean, yeah sure they weren't playing good, some players were clearly nervous and didn't want to risk forward passes... whistling their own team won't make these nervous players playing better!
The first goal of Sevilla started by a misplaced forward pass from Valencia, after 3-4 side-passes in the United defense while the whole stadium or a big part of it was whistling to them. To me it was clear that Valencia wanted to make a risky pass where he shouldn't, influenced by the heavy whistle that was going on at that time. The result was a rapid attack by Sevilla leading to a goal... I just won't and can't understand these fans that sabotage their own team, even if the team wasn't playing good at that time, if you guys don't want to support the team when they don't play good by cheering on them, at least don't whistle.
United exhibition made me sad, but I was also mad about the lack of support around the whole game, especially at the whistling.